Fix upgrade process for studentscanmark field.

On https://moodle.org/plugins/pluginversions.php?plugin=mod_attendance the previous version for a moodle 2.6 install is [2014022801]https://moodle.org/plugins/download.php/5996/mod_attendance_moodle26_2014022801.zip), and that version does *not* have a "studentscanmark" field, but 2014022802 does.  However, upgrade.php is checking for 2013082901 to add the studentscanmark field. This means that this upgrade path breaks things.